The demand for electro-chemical power cells, such as Lithium-ion batteries, is ever increasing due to the growth of applications such as electric vehicles and grid storage systems, as well as other multi-cell battery applications, such as electric bikes, uninterrupted power battery systems, and lead acid replacement batteries. It is a requirement for these applications that the energy and power densities are high, but just as important, if not more, are the requirements of low cost manufacturing and increased safety to enable broad commercial adoption. There is further a need to tailor the energy to power ratios of these batteries to that of the application.
For grid storage and electric vehicles, which are large format applications multiple cells connected in series and parallel arrays are required. Suppliers of cells are focused either on large cells, herein defined as more than 10 Ah (Ampere hours) for each single cell, or small cells, herein defined as less than 10 Ah. Large cells, such as prismatic or polymer cells, which contain stacked or laminated electrodes, are made by LG Chemical, AESC, ATL and other vendors. Small cells, such as 18650 or 26650 cylindrical cells, or prismatic cells such as 183765 or 103450 cells and other similar sizes, are made by Sanyo, Panasonic, EoneMoli, Boston-Power, Johnson Controls, Saft, BYD, Gold Peak, and others. These small cells often utilize a jelly roll structure of oblong or cylindrical shape. Some small cells are polymer cells with stacked electrodes, similar to large cells, but of less capacity.
Existing small and large cell batteries have some significant drawbacks. With regard to small cells, such as 18650 cells, they have the disadvantage of typically being constrained by an enclosure or a ‘can’, which causes limitations for cycle life and calendar life, due in part to mechanical stress or electrolyte starvation. As lithium ion batteries are charged, the electrodes expand. Because of the can, the jelly roll structures of the electrodes are constrained and mechanical stress occurs in the jelly roll structure, which limits its life cycle. As more and more storage capacity is desired, more active anode and cathode materials are being inserted into a can of a given volume which results in further mechanical stresses on the electrode.
Also, the ability to increase the amount of electrolyte in small cells is limited and as the lithium intercalates and de-intercalates, the electrode movement squeezes out the electrolyte from the jelly roll. This causes the electrode to become electrolyte starved, resulting in concentration gradients of lithium ions during power drain, as well as dry-out of the electrodes, causing side reactions and dry regions that block the ion path degrading battery life. To overcome these issues, especially for long life batteries, users have to compromise performance by lowering the state of charge, limiting the available capacity of the cells, or lowering the charge rate.
On the mechanical side, small cells are difficult and costly to assemble into large arrays. Complex welding patterns have to be created to minimize the potential for weld failures. Weld failures result in lowered capacity and potential heating at failed weld connections. The more cells in the array, the higher the failure risk and the lower manufacturing yields. This translates into higher product and warranty costs. There are also potential safety issues associated not only by failure issues in welds and internal shorts, but also in packaging of small cells. Proper packaging of small cells is required to avoid cascading thermal runaway as a result of a failure of one cell. Such packaging results in increased costs.
For large cells, the disadvantages are primarily around safety, low volumetric and gravimetric capacity, and costly manufacturing methods. Large cells having large area electrodes suffer from low manufacturing yields compared to smaller cells. If there is a defect on a large cell electrode, more material is wasted and overall yields are low compared to the manufacturing of a small cell. Take for instance a 50 Ah cell compared to a 5 Ah cell. A defect in the 50 Ah cell results in 10× material loss compared to the 5 Ah cell, even if a defect for both methods of production occurs at the same rate, in term of Ah produced between faults.
A jelly roll typically has one or more pair of tabs connecting to the cathode and anode current collectors, respectively. These are in turn connected to positive and negative terminals. The tabs generally extend a certain distance out from the jelly roll, which generates some void space in a cell, reducing energy density of the battery. Furthermore, for high power applications of Li-ion batteries, such as hybrid electric vehicles (HEV), high current drain is required. In this case, one pair of tabs may not be sufficient to carry the high current loading, as it will result in excessively high temperature at the tabs, causing a safety concern. Various solutions to address these issues have been proposed in prior arts.
U.S. Pat. No. 6,605,382 discloses multiple tabs for cathode and anode. These tabs are connected to positive and negative busbars. Since tabs are generally welded on cathode and anode current collectors, multiple tabs make jelly roll fabrication, particularly the winding process, very complicated, which increases battery cost. In addition, since the areas where the tab is welded onto the current collector has no active materials coating, the multiple tab configuration reduces energy of the battery.
To solve these issues caused by multiple tabs, solutions without tabs in a Li-ion jelly roll have been proposed in the patent literature and are currently used for high power Li-ion and ultra-capacitor cells. The core part of these solutions is to make a jelly roll with non-coated, bare cathode and anode current collector areas at both ends of the jelly roll and weld transition structural components at these ends to collect current.
U.S. Pat. No. 8,568,916 discloses transitional current collector components that take the form of Al and Cu discs. These discs are connected to positive and negative terminals through metal strip leads. Similar concepts have been disclosed and taught in U.S. Pat. Nos. 6,653,017, 8,233,267, US Patent Publn. No. 2010/0316897 and US Patent Publn. No. 2011/0223455. Although these disclosures may eliminate tabs from cathode and anode in a jelly roll, additional means to connect the positive and negative current collectors at the both ends of jelly roll to terminals are required, which still leaves void space in the cell, though less than in the conventional Li-ion cells having tabs. This compromises cell energy density. Furthermore, these solutions are only used in single jelly roll cells. U.S. Pat. No. 6,605,382 discloses a positive busbar where multiple cathode tabs are connected that is directly welded onto a disc which in turn is welded to an aluminum cylinder. This eliminates the need for a can bottom, reducing cell volume and weight. But the disclosure is only used for a multiple tab system.
A number of publications have disclosed means to build a large capacity unit by connecting multiple small cells in parallel. There is a challenge for these solution to properly arrange and configure cell tabs and busbars, and they suffer from low battery energy density, low power density, high cost and low safety. In U.S. Pat. No. 8,088,509, multiple jelly rolls are positioned in individual metal shells. The tabs from jelly rolls are connected to positive and negative busbars. In U.S. Pat. No. 5,871,861, a plurality of single jelly rolls are connected in parallel. Their positive and negative tabs are connected to positive and negative busbars. In WO 2013/122448, a Li-ion cell consisting of multiple jelly roll stacks formed by stacking cathode and anode plates is disclosed. The cathode tabs and anode tabs are connected to positive and negative busbars, respectively. In the foregoing prior art disclosures, multiple jelly rolls formed by winding or electrode stacking have multiple tabs and busbars and are housed in a metal casing.
In PCT/US2013/064654, new types of multi-core Li-ion structures have been disclosed. In one of these structures, a plurality of jelly rolls are positioned in a housing with liners for individual jelly rolls. Tabs from individual jelly rolls are connected to positive and negative busbars.
Another issue for large cells is safety. The energy released in a cell going into thermal runaway is proportional to the amount of electrolyte that resides inside the cell and accessible during a thermal runaway scenario. The larger the cell, the more free space is available for the electrolyte in order to fully saturate the electrode structure. Since the amount of electrolyte per Wh for a large cell typically is greater than a small cell, the large cell battery in general is a more potent system during thermal runaway and therefore less safe. Naturally any thermal runaway will depend on the specific scenario but, in general, the more fuel (electrolyte), the more intense the fire in the case of a catastrophic event. In addition, once a large cell is in thermal runaway mode, the heat produced by the cell can induce a thermal runaway reaction in adjacent cells causing a cascading effect igniting the entire pack with massive destruction to the pack and surrounding equipment and unsafe conditions for users.
For example, various types of cells have been shown to produce temperatures in the region of 600-900° C. in thermal runaway conditions [Andrey W. Golubkov et al, Thermal-runaway experiments on consumer Li-ion batteries with metal-oxide and olivin-type cathodes RSC Adv., 2014, 4, 3633-3642]. Such high temperatures may ignite adjacent combustibles, thereby creating a fire hazard. Elevated temperature may also cause some materials to begin to decompose and generate gas. Gases generated during such events can be toxic and/or flammable, further increasing the hazards associated with uncontrolled thermal runaway events.
Lithium ion cells may use organic electrolytes that have high volatility and flammability. Such electrolytes tend to start breaking down at temperatures starting in the region 150° C. to 200° C. and, in any event, have a significant vapor pressure even before break down starts. Once breakdown commences, the gas mixtures produced (typically a mixture of Co2, CH4, C2H4, C2H5F and others) can ignite. The generation of such gases on breakdown of the electrolyte leads to an increase in pressure and the gases are generally vented to atmosphere; however this venting process is hazardous as the dilution of the gases with air can lead to formation of an explosive fuel-air mixture that, if ignited, can flame back into the cell in question igniting the whole arrangement.
It has been proposed to incorporate flame retardant additives into the electrolyte, or to use inherently non-flammable electrolyte, but this can compromise the efficiency of the lithium ion cell [E. Peter Roth et al., How Electrolytes Influence Battery Safety, The Electrochemical Society Interface, Summer 2012, 45-49].
It should be noted that in addition to flammable gases, breakdown may also release toxic gases.
The issue of thermal runaway becomes compounded in batteries that include a plurality of cells, since adjacent cells may absorb enough energy from the event to rise above their designed operating temperatures and so be triggered to enter into thermal runaway. This can result in a chain reaction in which storage devices enter into a cascading series of thermal runaways, as one cell ignites adjacent cells.
To prevent such cascading thermal runaway events from occurring, storage devices may be designed to keep the energy stored sufficiently low, or employ enough insulation between cells to insulate them from thermal events that may occur in an adjacent cell, or a combination thereof. The former severely limits the amount of energy that could potentially be stored in such a device. The latter limits how close cells can be placed and thereby limits the effective energy density.
There are currently a number of different methodologies employed by designers to maximize energy density while guarding against cascading thermal runaway. One method is to employ a cooling mechanism by which energy released during thermal events is actively removed from the affected area and released at another location, typically outside the storage device. This approach is considered an active protection system because its success relies on the function of another system to be effective. Such a system is not fail safe since it needs intervention by another system. Cooling systems also add weight to the total energy storage system, thereby reducing the effectiveness of the storage devices for those applications where they are being used to provide motion (e.g., electric vehicles). The space the cooling system displaces within the storage device may also reduce the potential energy density that could be achieved.
A second approach employed to prevent cascading thermal runaway is to incorporate a sufficient amount of insulation between cells or clusters of cells that the rate of thermal heat transfer during a thermal event is sufficiently low enough to allow the heat to be diffused through the entire thermal mass of the cell, typically by conduction. This approach is considered a passive method and is generally thought to be more desired from a safety vantage. In this approach, the ability of the insulating material to contain the heat, combined with the mass of insulation required dictate the upper limits of the energy density that can be achieved.
A third approach is through the use of phase change materials. These materials undergo an endothermic phase change upon reaching a certain elevated temperature. The endothermic phase change absorbs a portion of the heat being generated and thereby cools the localized region. This approach is also passive in nature and does not rely on outside mechanical systems to function. Typically, for electrical storage devices, these phase change materials rely on hydrocarbon materials, such as waxes and fatty acids for example. These systems are effective at cooling, but are themselves combustible and therefore are not beneficial in preventing thermal runaway once ignition within the storage device does occur.
A fourth method for preventing cascading thermal runaway is through the incorporation of intumescent materials. These materials expand above a specified temperature producing a char that is designed to be lightweight and provide thermal insulation when needed. These materials can be effective in providing insulating benefits, but the expansion of the material must be accounted for in the design of the storage device.
In addition, during thermal runaway of lithium ion cells, the carbonate electrolyte which also contains LiPF6 salt, generally creates a hazardous gas mixture, not only in terms of toxicity but also flammability, as the gas includes H2, CH4, C2H6, CO, Co2, O2, etc. Such a mixture becomes particularly flammable when venting the cell to atmosphere. Indeed, when a critical oxygen concentration is reached in the mixture, the gas is ignited and can flame back into a cell, igniting the entire arrangement.
When comparing performance parameters of small and large cells relative to each other, it can be found that small cells in general have higher gravimetric (Wh/kg) and volumetric (Wh/L) capacity compared to large cells. It is easier to group multiples of small cells using binning techniques for capacity and impedance and thereby matching the entire distribution of a production run in a more efficient way, compared to large cells. This results in higher manufacturing yields during battery pack mass production. In addition, it is easier to arrange small cells in volumetrically efficient arrays that limit cascading runaway reactions of a battery pack, ignited by for instance an internal short in one cell (one of the most common issues in the field for safety issues). Further, there is a cost advantage of using small cells as production methods are well established at high yield by the industry and failure rates are low. Machinery is readily available and cost has been driven out of the manufacturing system.
On the other hand, the advantage of large cells is the ease of assembly for battery pack OEMs, which can experience a more robust large format structure which often has room for common electromechanical connectors that are easier to use and the apparent fewer cells that enables effective pack manufacturing without having to address the multiple issues and know-how that is required to assemble an array of small cells.
In order to take advantage of the benefits of using small cells to create batteries of a larger size and higher power/energy capability, but with better safety and lower manufacturing costs, as compared to large cells, assemblies of small cells in a multi-core (MC) cell structure have been developed.
One such MC cell structure, developed by BYD Company Ltd., uses an array of MC's integrated into one container made of metal (Aluminum, copper alloy or nickel chromium). This array is described in the following documents: EP 1952475 A0; WO2007/053990; US2009/0142658 A1; CN 1964126A. The BYD structure has only metallic material surrounding the MCs and therefore has the disadvantage during mechanical impact of having sharp objects penetrate into a core and cause a localized short. Since all the cores are in a common container (not in individual cans) where electrolyte is shared among cores, propagation of any individual failure, from manufacturing defects or external abuse, to the other cores and destruction of the MC structure is likely. Such a cell is unsafe.
Methods for preventing thermal runaway in assemblies of multiple electrochemical cells have been described in US2012/0003508 A1. In the MC structure described in this patent application, individual cells are connected in parallel or series, each cell having a jelly roll structure contained within its own can. These individual cells are then inserted into a container which is filled with rigid foam, including fire retardant additives. These safety measures are costly to produce and limit energy density, partly due to the excessive costs of the mitigating materials.
Another MC structure is described in patent applications US2010/0190081 A1 and WO2007/145441 A1, which discloses the use of two or more stacked-type secondary batteries with a plurality of cells that provide two or more voltages by a single battery. In this arrangement, single cells are connected in series within an enclosure and use of a separator. The serial elements only create a cell of higher voltage, but do not solve any safety or cost issues compared to a regularly stacked-type single voltage cell.
These MC type batteries provide certain advantages over large cell batteries; however, they still have certain shortcomings in safety and cost. In addition, from the point of increasing Li-ion battery energy density, reducing cost and improving safety, it is desirable, for lowered cost and higher performance, to (i) eliminate tabs and liners, (ii) integrate both positive current collectors and positive busbars together, (iii) integrate both negative current collectors and negative busbar together and (iv) allow a quick heat depletion at the positive current collector and busbar.